The present invention relates generally to permanent magnet rotors, and more particularly to novel techniques for retaining a core and a plurality of magnetic elements in assembled relation.
Permanent magnet rotors for dynamuelectric machines, such as electronically commutated motors (ECMs), typically are constructed of a generally cylindrical iron core, which may be of a solid or sintered metal construction or may be a laminated core, and around which are positioned a plurality of magnetizable elements. The magnetizable elements are arcuate shaped with an inner contour conforming to the outer surface of the core such that the magnets may be adhered to the core by an adhesive layer. The magnetizable elements are generally made from barium or strontium ferrite, or samarium cobalt (rare earth). Permanent magnet rotors of this general type find particular application in motors having control circuitry which controls energization of the windings in one or more predetermined sequences to provide rotational magnetic fields and thereby rotation of the rotor. Numerous patents disclosing such rotor applications are identified in copending U.S. patent application Ser. No. 459,633, filed Jan. 2, 1990, entitled "Permanent Magnet Rotor and Method and Apparatus for Making Same", which is assigned to the assignee of the present invention and incorporated herein by reference.
The relative high mass of the materials used to form the magnetic elements and the relatively high rotor speeds, generally in the order of 1000-16,000 RPM, create significant forces during rotor operation, including centrifugal forces and substantial momentum forces upon sudden reversals of rotor rotation. Retention of the magnets in predetermined rotational positions about the core is critical to optimum motor operation. A number of methods and techniques for retaining magnets in fixed relation on the rotor core have previously been considered. One such technique involves positioning the magnetic elements around the core and applying a Kelvar or fiberglass wrap to hold the magnets in place. In this technique, an adhesive filler may be employed to fill voids and provide a more rigid structure, thereby further preventing magnet movement about the core during motor operation.
Another technique for retaining magnets on an associated core in a permanent magnet rotor includes wrapping a relatively fine wire, under tension, around the magnetic elements followed by an adhesive or epoxy coating to protect the wire and "pot" the assembly. Another technique employs a cylindrically shaped shell or "can" which is assembled around the outer peripheral surfaces of the magnets. The various prior methods and techniques for retaining magnets in fixed rotational position on associated cores in permanent magnet rotors are described in greater detail in the aforementioned copending U.S. patent application Ser. No. 459,633.
Still another technique provides machined or cast pockets on the rotor core to receive and retain the magnets in fixed rotational relation on the rotor. See, for example, U.S. Pat. Nos. 4,549,341 (Kasabian) dated Oct. 29, 1985; No. 4,617,726 (Denk) dated Oct. 21, 1986; and No. 4,625,135 (Kasabian) dated Nov. 25, 1986. Other techniques for retaining magnets in fixed relation on rotor cores include providing mutually engaging flat surfaces on the cores and magnets as disclosed in U.S. Pat. Nos. 4,656,379 (McCarty) dated Apr. 7, 1987; No. 4,631,435 (McCarty) dated Dec. 23, 1986; and No. 4,633,113 (Patel) dated Dec. 30, 1986. Yet another technique employs wedge shaped magnets which are retained in relatively complex pockets or slots formed in the rotor as exemplified by U.S. Pat. Nos. 4,332,079 (Silver) dated Jun. 1, 1982; No. 4,339,874 (McCarty) dated Jul. 20, 1982; and No. 4,336,649 (Glaser) dated Jun. 29, 1982.
Other techniques which have been employed to secure magnets to rotors include the use of adhesives, such as disclosed in U.S. Pat. No. 3,531,670 (Loudon) dated Sep. 29, 1970; and the use of a cast aluminum core as disclosed in U.S. Pat. No. 4,088,177 (Armstrong et al.) dated May 9, 1978.
While many of the approaches to retaining magnets on rotor cores exemplified by the aforementioned patents have been found satisfactory for their intended applications, many exhibit drawbacks such as difficulty in manufacture and/or assembly, or failure to adequately retain the magnets in fixed rotational position on the rotors during high speed operation and when subjected to repeated starting, stopping and reversal of the motor in which the rotor is used.
Still another technique for preventing slippage of magnets relative to their cores in permanent magnet rotors is to provide spacers or projections on the exterior of the core between adjacent magnets as disclosed in U.S. Pat. No. 4,742,259 (Schaefer). However, with this technique, the magnets may be forced against the projections under operational stress so that flaking or breakage of the magnets may result, primarily at the sharp corner edges of the magnets adjacent the core. This problem may be more serious with molded or ceramic magnets which tend to chip with the possibility of magnet particles escaping into other parts of the motor and causing damage or even failure of the motor during operation.